In addition to firefox I frequently use midori as a browser. Normally I'm quite satisfied with it. Yet there's one thing that keeps annoying me. Whenever I want to watch a video on youtube I can only get the audio. But there's no video at all. As far as I know midori does support HTML5, so theoretically everything should work.
Any suggestions are welcome.

Midori uses GStreamer for HTML5 audio and video support. Codecs, which handle particular formats of audio or video, are provided by GStreamer plugins which may need to be installed separately. Midori may be built with GTK+2 or GTK+3 (visit about:version to check), which correspond to GStreamer versions of 0.10 or 1.0 respectively.

You need gstreamer(0.10/1.0)-pulse if you're using PulseAudio.
You may need gstreamer(0.10/1.0)-alsa for ALSA, depending on your distribution.
You need plugins for Theora, gstreamer(0.10/1.0)-base and MPEG-4 incluing aac (e.g. gst-plugins-faad), gstreamer(0.10/1.0)-bad. For WebM, you'll need plugins for vorbis (-base), matroska (-good), and vp8 (-bad). Have a look at http://www.gstreamer.net/documentation/plugins.html for details.
For Youtube or Vimeo, you need WebKitGTK+ 1.1.20 or newer.
You can test your installed codecs here..
You can look at about:version to see which video codecs you have installed.